1. Field of the Invention
The present invention generally relates to industrial emission control systems and, more particularly, is concerned with an apparatus and process for removing and destroying volatile organic compounds by heating adsorbent material with a humidified flue gas to release sorbed compounds whereafter the compounds are burned. The present invention also relates to a method for controlling the regeneration of an adsorbent material by monitoring a regeneration variable such as flue gas temperature and, based thereon, controlling the burning of an outside fuel in combination with desorbed volatile organic compounds.
2. Description of the Prior Art
A daily news barrage about the health hazards of living in an industrialized world, hazards due to such things as urban smog, acid rain and ozone depletion, has heightened our awareness of the fragile ecosphere mankind inhabits. The uneasiness of the general public with respect to destruction of the environment has led national and state governments to revitalize scrutiny of industrial pollution. Airborne emissions in industrial environments are typically limited by considerations of personnel or equipment protection, public nuisance, government regulations and economics. Because the former two considerations are often anticipated by government regulations, it is principally the latter two considerations which influence the design of emission control systems. Unfortunately, regulations and economics are typically at odds with one another and therefore must be balanced. With respect to the present invention, the tradeoffs in specifying emission control systems for the removal of volatile organic compounds from contaminated gas streams are challenging, and they effect many segments of our economy.
Volatile organic compounds (VOCs) include hydrocarbons such as ethers, esters, ketones, aldehydes and alcohols. Industrial plants which vent these types of organic compounds, or solvents, into the atmosphere are coming under increasingly stringent emission standards, such as those recently enacted by the state of California. These regulatory and economic burdens are shared by a spectrum of industrial users including, for example, semiconductors, paint manufacturing, fiberglass molding operations, dry cleaning, auto and appliance paint baking and textiles, to name just a few.
In such industries, recycling and direct incineration of VOCs are completely satisfactory options for reducing VOC emissions. However, due to low volumetric ratios of solvent vapors in gas streams, reuse and resale of recovered solvents is not always economically feasible. Direct incineration of these vapors may also be impractical because of low VOC concentrations in conveying gas streams.
The most satisfactory solution for controlling industrial VOC emissions, however, seems to focus on a two-step process of first concentrating the solvent vapors with adsorbent materials such as activated carbon, and then regenerating the adsorbent materials, burning off the concentrated vapors in the process. The adsorption of volatile organic compounds relies on blowing a contaminated gas stream across, or through, a carbon bed prior to releasing the stream into the atmosphere. Over time, the carbon bed becomes saturated with VOCs and therefore must be either replaced or regenerated. When a carbon bed is regenerated, the concentrated VOCs are "boiled" off and then the activated carbon is reused. Regeneration of an activated carbon adsorbent is best accomplished by heating the carbon bed to around 280.degree. F. at steady state. In most present systems the bed is heated with hot air. The concentrated sorbed hydrocarbons thus released are then destroyed by burning in a combustion unit, usually in combination with an outside fuel.
Since adsorption and regeneration are independent steps, VOC contaminated gas streams contacting an adsorbent cannot be filtered for VOCs concurrently with regeneration of the same adsorbent. Instead, each adsorbing cycle is followed by a regenerating cycle. Therefore, most present emission control systems employ two adsorbent containers, or adsorbers, connected to two plumbing systems corresponding to the adsorption and regeneration cycles. With this configuration, one self-contained system in on-line adsorbing mode can operate in tandem with another self-contained system in off-line regeneration mode. After sufficient operating time, to allow accumulation of VOCs in the adsorbent material of the on-line adsorber, valves are used to redirect the plumbing systems between the two adsorbent containers and afterwards, the on-line adsorber becomes the off-line adsorber and visa versa.
An example of one such volatile organic compound emission control system is disclosed by Mattia (U.S. Pat. No. 3,455,089). The Mattia patent shows a portion of an enriched regeneration gas being passed as a slipstream to either an air incinerator or catalytic combustion chamber, therein to be burned. Natural gas may be added to the enriched regeneration gas if combustion is not otherwise possible. To release sorbed compounds, outside air, or make-up regeneration air, is heated with a heat exchanger prior to contacting an adsorbent material. Steam may be introduced to control the humidity of the make-up regeneration air.
While the Mattia emission control system, or other variations, have been used successfully in the past, the technique of using outside air and steam to heat adsorbents has its shortcomings. First, in these systems, outside air is heated with heat exchangers which derive heat from combustion gases generated from an air incinerator or catalytic combustion chamber. Heat exchangers add cost and bulk to the overall emission control system. Second, using steam to humidify the outside air shares a common problem with scrubbers: the water which condenses is polluted, and such effluent must itself be safely disposed of. Furthermore, steam in present VOC emission control systems is generated by boilers, system components which in turn must be fueled. Moreover, steam temperatures above 212.degree. F. are only achieved by superheating, which requires a pressurized system at dramatically increased cost. Third, lacking from present emission control systems is a way of combining the primary advantage of hot air, to heat the adsorbent to temperatures above 212.degree. F., with the primary advantage of steam, quick adsorbent heat-up.
Consequently, a need exists for improvements in adsorbent type emission control systems for volatile organic compounds which will result in eliminating heated outside air and heat exchangers, safely and inexpensively using water to humidify hot air, eliminating inefficiencies caused by generating steam, and providing a quick adsorbent heat-up to temperatures above 212.degree. F.